November 2004

New lasers bring new precision for APOL group

"We have our hands full with a lot of different tasks at the moment," says Alan Fried of the Analytical Photonics and Optoelectronics Laboratory (APOL). That's putting it mildly: Alan and his colleagues—Dirk Richter, Jim Walega, Petter Weibring, and Chad Roller—are developing state-of-the-art trace gas detection systems while maintaining, operating, and improving on the current generation of airborne tunable diode laser absorption systems, not to mention analyzing and writing up results from earlier field projects.

Left to right: Peter Weibring, Jim Walego, Dirk Richter, and Alan Fried of the APOL Group.

APOL was born a few years ago with funding from NSF to develop spectrometers based on telecommunication lasers and optical fiber technology, both relatively new tools to atmospheric science. It was fueled by previous and ongoing collaborations between Alan and Frank Tittel, a professor at Rice University. Frank and his colleagues leveraged the growth of off-the-shelf lasers from the telecommunications industry to develop much more sophisticated lasers directly applicable to atmospheric research.

These new lasers are based on a process known as difference frequency generation (DFG), a nonlinear optical mixing technique that results in the generation of light at new wavelengths that are important in atmospheric research. A few years ago, Alan and Frank wrote a number of joint proposals to further explore DFG technology, with emphasis on its application to atmospheric measurements. One proposal exploits this new technology to measure carbon dioxide isotopic ratios (13 CO 2 / 12 CO 2) with very high precision. Alan says, "We saw a real need to develop portable field instrumentation for such studies." NSF funded the proposal, and the new lab became a joint effort between ACD and ATD.

Although APOL is somewhat new to NCAR, Alan isn't; he has been using tunable diode lasers to study atmospheric trace gases for over a decade, well before DFG technology was feasible for this purpose. The current generation, called the Dual-Channel Airborne Laser Spectrometer (DCALS), is flown on research aircraft to measure atmospheric concentrations of formaldehyde, which can be as small as 10-50 parts per trillion. DCALS and previous versions of the instrument have been used in field projects worldwide, producing many valuable data sets. Most recently, Alan and Jim, with help from Petter and Chad, successfully acquired ambient formaldehyde measurements during NASA's Intercontinental Chemical Transport Experiment—North America study (INTEX-NA) to measure pollution emanating from the North American continent and directed out over clean, remote regions of the North Atlantic Ocean. "Jim's extensive experience in airborne measurements was a great asset to this effort," says Alan.

Formaldehyde is formed in the atmosphere from the oxidation of most human-made and natural hydro-carbons, and it decomposes to produce carbon monoxide and reactive hydrogen radicals. These hydrogen radicals are extremely important in controlling the oxidizing capacity of the atmosphere and in the formation of ozone. Comparing atmospheric formaldehyde concentrations with modeled concentrations allows scientists to check their understanding of the chemical reactions that take place as pollutants break down. Using their airborne formaldehyde measurements, Alan and the APOL group have worked closely with modelers at NCAR, NOAA, NASA, and the universities to improve our understanding of hydrocarbon oxidation mechanisms in the atmosphere (see On the Web).

Despite their success in measuring trace formaldehyde levels as low as 30 parts per trillion in the atmosphere with tunable diode lasers, Alan, Dirk, and their colleagues early on recognized that the DFG method could be exploited to measure formaldehyde and other trace gases at even lower concentrations. Systems using new DFG laser sources are significantly smaller and lighter than the current tunable diode laser systems; additionally, they can be operated at room temperature, whereas the tunable diode lasers have to be cooled to liquid nitrogen temperatures. The new systems also promise to be significantly more stable and robust. These are all important attributes for airborne instruments. Moreover, the new generation of airborne DFG laser systems the APOL group is developing will operate on NCAR's new HIAPER airplane in the future without the need for an in-flight operator.

The DFG system uses lasers that are employed by the telecommunications industry, which has invested many millions of dollars in their development. These telecom devices emit laser radiation in the near-infrared spectral region (1-1.5 micrometers), where absorption from most trace atmospheric gases is extremely weak. However, under the appropriate conditions, the output of two such near-infrared laser beams can be mixed in a nonlinear crystal (lithium niobate, the same crystal that is found in cell phones) to generate light in the mid-infrared region of 3-5 micrometers, where trace gas absorption is highest.

Until recently, this mixing process as well as the difficulty in selecting proper laser types resulted in extremely low output powers that were insufficient for atmospheric research. Over the past few years, however, Frank and his Rice colleagues advanced DFG sources using telecom lasers and optical fibers to a point at which they sparked renewed interest, and this resulted in collaboration with the NCAR group. Dirk, formerly a graduate student in Frank's group, joined NCAR as a postdoctoral fellow to further advance DFG development, and in the process he has led the development of some very innovative new designs. Petter, a postdoctoral fellow from the Lund Institute in Sweden, joined the APOL group last December and has enhanced and accelerated the DFG development even further. With a strong background in optics and signal processing, Petter has successfully implemented new strategies to circumvent a number of performance-limiting roadblocks in DFG development. As a result, the APOL group now routinely achieves performance with their DFG system that surpasses that of the significantly larger tunable diode laser systems. They anticipate that the system's performance, which is already unmatched by that of any other group in the world, will soon be improved even further.

Alan points out another advantage of DFG laser systems: the use of off-the-shelf components eliminates the current requirement to have a laser tailormade for each needed wavelength region. Dirk adds that because such devices are fiber coupled, scientists can readily exchange wavelengths or add new ones by fusion-splicing lasers in and out, a process that takes a skilled person only a few minutes.

The APOL group is building two field DFG instruments. One will replace the DCALS system to measure formaldehyde on airborne platforms, and the other will measure carbon dioxide isotopic ratios. The latter work is funded by an NSF Biocomplexity grant. During photosynthesis, plants preferentially take in and process the lighter isotope, 12 CO 2 , leaving more of the heavier isotope in the atmosphere. This is in contrast to oceanic uptake of CO 2, which hardly changes the ratio of the two isotopes. High-precision measurements of the ratio of the two isotopes will help scientists discern the magnitude of these two sink processes globally. Currently, the best way to make this measurement is by high-precision mass spectrometry, but the instrument can't be moved out of the laboratory. The new laser-based instrument may not be quite as precise as the lab one, but it will collect a significantly larger amount of data by eliminating the time-consuming job of collecting samples and returning them to the lab to be analyzed.

There's a large educational component to the biocomplexity work: two Front Range science teachers and two students were chosen to work in the APOL lab with the NCAR scientists and to develop materials on the carbon cycle for secondary school classes. Their Web site includes interviews with NCAR scientists on the carbon cycle, climate change, and the carbon-isotope measuring system. In addition, Chad Roller, a graduate student from Frank's laboratory at Rice, has been working on this project with the APOL scientists.

The APOL group expects to have an airborne DFG laser system for formaldehyde ready for flight-testing next year. Because of some significant recent advances in DFG development, the group points out that they're no longer inhibited by problems with fundamental laser physics but only by more tractable engineering issues. If everything works as planned, they'd like to fly the new instrument on three major field programs in 2006. With that busy future lined up for the DFG laser system, it looks like the group is going to keep having its fingers in a lot of tasks for a long time.